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intermittent cable modem resets caused by signal drop

Hello all

Here's the lengthy status of my current problems with cox cable modem dropping.

Cable modem SB6121 will lose connectivity and reset, going down to one light, then rebuilding the connection after about a one minute outage.  Happens regularly and randomly.

Here's an example of the logs found in the motorola cable modem.  If you haven't checked your own cable modem logs and you're having this problem, here's the link to prove you're having the same problem as me: .  I xxx'd out my MAC addresses in the logs.

Jun 20 2012 01:57:47 6-Notice I401.0 TLV-11 - unrecognized OID;CM-MAC=xxxx;CMTS-MAC=xxxx;CM-QOS=1.1;CM-VER=3.0;
Jun 20 2012 01:57:46 5-Warning D04.1 ToD request sent - No Response received;CM-MAC=xxxx;CMTS-MAC=xxx;CM-QOS=1.1;CM-VER=3.0;
Jun 20 2012 01:57:34 3-Critical R02.0 No Ranging Response received - T3 time-out;CM-MAC=xxx;CMTS-MAC=xxx;CM-QOS=1.1;CM-VER=3.0;
Jun 20 2012 01:57:30 4-Error T201.0 Missing Mandatory MDD TLV on primary DS Channel;CM-MAC=xxx;CMTS-MAC=xxx;CM-QOS=1.1;CM-VER=3.0;

When it comes to signal strength, this is apparently not the problem.  

Here's what motorola says should be fine, according to Motorola Tech Support:

Downstream Signal to Noise Ratio (SNR): 25 dB or greater
Downstream Power Level: +15 to -15dBmV
Upstream Transmit Power Level: less than 58dBmV

My values, pasted from  I've sampled these values over time and they have never strayed much from these numbers.

downstream Signal to Noise Ratio 38 dB  38 dB  38 dB  38 dB 
downstream Power Level 1 dBmV   0 dBmV   0 dBmV   0 dBmV
upstream Power Level 44 dBmV

After multiple lengthy calls with Motorola Tech Support, who are surprisingly helpful, they told me that this problem is definitely at the service provider end (Cox), with the signal dropping out on the modem.  They offered for free to replace my old Motorola SB6120 with a  new SB6121, saying they're only doing this to prove that the cable modem isn't the problem, and to aide my case for Cox to fix it.

Now, I've been receiving these random restarts (which sometimes occur in bursts of 3-4 per hour) since I lived at my prior residence, three blocks away from my new place in the same neighborhood.

So, in summary.

Cable modem random restarts resulting in 1 minute outages.

Two different cable modems.

Two different HOUSES in the same neighborhood.

Signal strength to the cable modem is NOT a problem.

What does Cox do?

Schedule another technician to come out.  The last scheduled appointment on Thursday from 5-7pm didn't call, didn't show.  Got another one scheduled on Monday.  We'll see.

I'm posting this because if anyone else has the same problems, they should check their Motorola cable modem logs and signal strengths and post here.

  • FYI I am in Baton Rouge, Louisiana.

  • i had the exact same issue... had 7 cox techs come out... none could find issue... bought a brand new motorola 6121 and had the same issue...  finally I tried Cox's new Cisco DPC3010 (I think it was $70 or $80 and it fixed the problem entirely... cox and motorola aren't getting along and they will not acknowledge that fact... you can get one of the cox techs to loan you the modem to try it for a few days pretty easily... if you talk to cust. service you can probably get them to GIVE you the modem as compensation for your downtime...

  • Have the same issues but I have the SB6120. Will post my logs here shortly.

  • That is very interesting and explains why multiple houses and multiple cable modems keep having the same problems.  I can only assume that Cox's problems with Motorola are Cox's fault, otherwise there would be more of a public "stay away" message.  Cox phone support people recommend the Motorola 6121 first and foremost, so I have trouble thinking that is the problem.  

    What this does NOT explain is why the 6120 and 6121 work fine MOST of the time, for long stretches even.  

    My guess would be that Cox experiences this issue when localized demand has outgrown capacity, but I suppose we'll never know.

  • Cox technician came today and said that he sees this problem once a week - the new Docsis 3.0 modems "range" to different frequencies when more bandwidth is detected on another frequencies.  During this "ranging" the signal drops.  He said it happens with all brands of cable modems.

    The technician said that the "engineers" back at the office can "fix" this.  He said he didn't really know how.  I asked if it was essentially locking the modem into one frequency, and he said yes.  He checked the signal, as he had to do at every visit, and said I had a good signal.  He took my information, said he'd be glad to call me back and let me know when "she" back at the office "fixes it". I appreciated that gesture at least.  

    I'm not sure what/if anything to believe from Cox, other than they seem incapable of delivering clear information to the general public regarding their troublesome service.  Instead of treating cable signals like "magic" it would make more sense to deliver clearer information, so as to remove superstition and prevent silly things like me buying a new cable modem and spending hours on the phone with Motorola to fix mysterious problems.